Herbert A. Simon, whose curiosity about how people make decisions helped lay the groundwork for such fields as artificial intelligence and cognitive psychology and won him the 1978 Nobel Prize in economics, died yesterday at age 84.

Computer expert and Nobel Prize winner Dr. Herbert A. Simon in his office at Carnegie Mellon University in March 1986. (Post-Gazette)

Until just a few weeks ago, Dr. Simon had been in good health, continuing to pursue his research at Carnegie Mellon University, publishing papers and seeing students.

But in mid-January, he underwent surgery at UPMC Presbyterian to remove a cancerous tumor from his abdomen. The surgery was successful and the care he received was exquisite, said his daughter, Katherine Simon Frank, but he then suffered a series of what turned out to be fatal complications.

"Obviously, we're all talking a lot about him today around here," said psychology professor Kenneth Kotovsky. "One of the joys of my existence ... has been that his office is right down the hall from me. Running into him in the hall would often lead to an amazing intellectual discussion. You could talk to him about anything."

Dr. Simon had been a fixture for 52 years at Carnegie Mellon, a university that he helped transform and make famous. He and the late Allen Newell gained renown in the mid-1950s when they created the first "thinking machine" and launched the field of artificial intelligence. Both also were central figures during the cognitive revolution in psychology in the 1960s as scientists began to use computer models to study human thought processes.

As a scientist and later a university trustee, Dr. Simon played key roles in creating the computer science department and the Robotics Institute and founding the cognitive science group within the psychology department. The Graduate School of Industrial Administration and the departments of social and decision sciences, philosophy, statistics and physics all bear his imprint.

"Few, if any scientists and scholars in the world have had as great an influence as has Herb across so many fields -- economics, computer science, psychology and artificial intelligence among them," said university President Jared L. Cohon.

Dr. Simon's presence attracted scores of other researchers and untold numbers of students to the university, making him arguably one of the most powerful figures on the campus. His influence was so great, suggested psychology professor David Klahr, "it should be named Carnegie Mellon Simon University."

Born in Milwaukee in 1916, he was the son of an electrical engineer-turned-patent attorney; his mother was an accomplished pianist.

"I like to think that since I was about 19, I have studied human decision-making and problem-solving," Dr. Simon said in a Post-Gazette interview last fall. He earned a doctorate in political science at the University of Chicago in 1943 and took teaching positions at the Illinois Institute of Technology and the University of California, Berkeley, before joining the newly established industrial administration school at the Carnegie Institute of Technology.

He had hoped to use mathematics to give the social sciences the same rigor as such hard sciences as physics and chemistry, but found that a frustrating experience; even with the new machine called a computer that was available at Carnegie Tech, it seemed that something was always missing when human factors were translated into numbers.

In the mid-'50s, however, he came to realize that computers weren't just number-crunchers, but could be used to study patterns of any type. "I realized that you could formulate theories about human and social phenomena in language and pictures and whatever you wanted on the computer and you didn't have to go through this straitjacket adding a lot of numbers," he said in his Post-Gazette interview.

During Christmas break in 1955, he, Newell and programmer J.C. Shaw made that vision a reality by creating Logic Theorist, a computer program that could discover proofs of geometric theorems. It was the first computer program capable of thinking and marked the beginning of what would become known as artificial intelligence.

In 1957, Dr. Simon became convinced that computers not only could think, but that a computer would be able to beat the world's best chess player within 10 years. It was a prediction that would later come back to haunt him; it actually was 40 years before IBM's Deep Blue would win the chess championship from Garry Kasparov.

But Raj Reddy, former dean of computer science, said that prediction really wasn't far off. "It was a do-able thing in 10 years," he said. "But it was like putting a man on the moon. You had to devote resources to it and no one did then."

His infatuation with human decision-making would lead Dr. Simon across other academic disciplines. He and Newell developed a theory that the human mind manipulates symbols in much the same way that a computer does. They were at the core of the cognitive revolution in the 1960s, when psychologists began to use computers to study how the thinking process works.

Most notably, his quest to understand decision-making led him to develop his economic theory of "bounded rationality," for which he was awarded the 1978 Nobel in economics.

Classical economists had argued that people make rational choices to obtain the best commodity at the best price. Dr. Simon said that was impossible -- too many choices and too little time to analyze them cause people to choose the first option that is good enough to meet their needs.

Dr. Simon once wrote that the best choice he ever made was to convince Dorothea Pye to marry him on Christmas Day 1937.

In his 1991 autobiography, "Models of My Life," Dr. Simon said he feared that his dedication to his work, which often entailed 60- to 80-hour weeks, had made him a less than perfect father. But his daughter, Katharine, didn't see it that way. "I respected the fact that he was so devoted to his work. I always viewed him as a model to be followed."

She remembers that the family always had meals together, despite her father's long hours, and that he always led them in lively conversations. Adding to the conversational milieu were her father's graduate students, who were often invited to dinner.

One of those students in the early '60s was Klahr, who remembers his mentor as both brilliant and intimidating. "He always assumed that the people he was talking to knew as much as he did -- and they rarely did." And he seemed to know everything, conversing as easily about astrophysics as his own field of cognitive psychology.

Kotovsky, another former graduate student, said Dr. Simon loved to argue. When he would preface a statement with the words, "Look, friend ...," that was a signal that he was about to put the kibosh on his opponent's argument.

"You had to be sure your head was attached when he used the word 'friend,' " said Kotovsky. He recalled the first time Dr. Simon directed "Look, friend" his way: "That was the moment I passed into adulthood."

Dr. Simon enjoyed playing the piano and, particularly in recent years, used to gather with friends who played violin, viola and other instruments.

In addition to the Nobel, Dr. Simon was the recipient of virtually every top award in every scientific field he pursued: the A.M. Turing Award in computer science, the American Psychological Association Award for Outstanding Lifetime Contributions to Psychology, induction into the Automation Hall of Fame, the American Society of Public Administration's Dwight Waldo Award and the National Medal of Science, among them.

He was always appreciative of such honors, but maintained they were no big deal. "The thing that he really cherished was doing his job as a professor," Kotovsky said.

One night, for instance, Kotovsky had invited the Nobel laureate to speak to a group of freshmen at one of the residence halls.

After Dr. Simon spoke, everyone sat on the floor eating submarine sandwiches, while the students huddled around him. The conversation continued for hours until Kotovsky, worried that Dr. Simon might be getting impatient and tired, sidled up and asked, "Will you be ready to leave soon?"

"No, you go on," Dr. Simon replied. "I'll be fine here."

"That," Kotovsky added, "was who he was."

He is survived by his wife, Dorothea; two daughters, Katherine Simon Frank of Minneapolis, Minn., and Barbara M. Simon of Wilder, Vt.; and a son, Peter Simon of Bryan, Texas.

Visitation will be from 2 to 4 p.m. and from 7 to 9 p.m. tomorrow at the Samson Funeral Home, 537 N. Neville St., Oakland. Burial will be private.

The family suggests memorial contributions to Carnegie Mellon or the First Unitarian Church of Pittsburgh, Morewood and Ellsworth avenues, Pittsburgh, 15213.